1. Field of the Invention
The present invention relates generally to the field of optical communications systems. More specifically, the present invention discloses an optical wavelength router for wavelength division multiplex (WDM) optical communications.
2. Statement of the Problem
Wavelength division multiplexing is a commonly used technique that allows the transport of multiple optical signals, each at a slightly different wavelength, on an optical fiber. The ability to carry multiple signals on a single fiber allows that fiber to carry a tremendous amount of traffic, including data, voice, and even digital video signals. As an example, the use of wavelength division multiplexing permits a long distance telephone company to carry thousands or even millions of phone conversations on one fiber. By using wavelength division multiplexing, it is possible to effectively use the fiber at multiple wavelengths, as opposed to the costly process of installing additional fibers.
In wavelength division multiplexing techniques, multiple wavelengths can be carried within a specified bandwidth. It is advantageous to carry as many wavelengths as possible in that bandwidth. International Telecommunications Union (ITU) Draft Recommendation G.mcs, incorporated herein by reference, proposes a frequency grid which specifies various channel spacings including 100 GHz and 200 GHz. It would be advantageous to obtain 50 GHz spacing. Separating and combining wavelengths with these close spacings requires optical components which have high peak transmission at the specified wavelengths and which can provide good isolation between separated wavelengths.
One technique which has been developed to accomplish the demultiplexing of closely spaced wavelengths is to cascade a series of wavelength division demultiplexing devices, each device having different wavelength separating characteristics. A typical application involves cascading an interferometric device such as an arrayed waveguide device having a narrow spacing of transmission peaks (e.g., 50 GHz) with a second interferometric device which has a coarser spacing and correspondingly broader transmission peaks (e.g., 100 GHz spacing). The cascade of devices provides the separation of wavelengths by subdividing the wavelengths once in the first device, typically into a set of odd and even channels, and then separating wavelengths in the subsets in following devices in the cascade.
Arrayed waveguide, fused biconical taper, fiber Bragg grating, diffraction grating, and other interferometric wavelength demultiplexing devices can be constructed to have the appropriate characteristics for the first or second stage devices in the cascade. However, traditional interferometric devices have the characteristic that as the spacing of the channels is decreased, the transmission peaks become narrower, and are less flat over the wavelength region in the immediate vicinity of each peak than a device with wider channel spacings. As a result, when using a traditional device in the first stage of a cascade, the transmission peaks may not have a high degree of flatness, and any drift or offset of a wavelength from its specified value may result in significant attenuation of that wavelength. In addition, the isolation between wavelengths is frequently unsuitable with conventional interferometric devices and can result in unacceptable cross-talk between channels.
With increasing numbers of wavelengths and the close wavelength spacing which is utilized in dense wavelength division multiplexing systems, attenuation and cross-talk must be closely controlled to meet the system requirements and maintain reliable operations. As an example, 40 or 80 wavelengths can be generated using controllable wavelength lasers, with transmission signals modulated onto each laser. It is desirable to be able to demultiplex these channels. Although the lasers can be controlled and the wavelengths stabilized to prevent one channel from drifting into another, there is always some wavelength drift which will occur.
For the foregoing reasons, there is a need for a wavelength division demultiplexing device which tolerates wavelength drift, maintains a high degree of isolation between channels, and is able to separate large numbers of wavelengths.
3. Prior Art
FIG. 1 illustrates a prior art interferometer that shares some of the basic principles employed in the present invention. An input laser beam is split into two beams by a beamsplitter 10. One beam propagates toward a mirror 14 and is reflected back by this mirror. The other beam propagates toward a resonator 12 and is also reflected back. The resonator 12 is a Fabry-Perot cavity with a partially-reflective front mirror and a totally-reflective back mirror. The resonator 12 reflects substantially all of the incident optical power back regardless of wavelength, but the group delay of the reflected light is strongly dependent on wavelength. The two reflected beams from the mirror 14 and from the resonator 12 interfere at the beamsplitter 10 and the resulting output is split into two beams, one at output A, and the other in a different direction at output B. The two output beams contain complementary subsets of the input optical spectrum, as shown for example in FIG. 2. Such a wavelength router concept has been proposed by B. B. Dingle and M. Izutsu, "Multifunction Optical Filter With A Michelson-Gires-Tournois Interferometer For Wavelength-Division-Multiplexed Network System Applications," Optics Letters, vol. 23, p.1099 (1998) and the references therein.
The two output ports A and B divide the spectral space evenly with alternating optical channels being directed to each output port (i.e., optical channels 1, 3, 5, 7, etc. are directed to output port A, while channels 2, 4, 6, etc. are directed to output port B). This function has sometimes been called an optical interleaver.
4. Solution to the Problem
The present invention address the problems associated with the prior art using a polarization-based interferometer to implement an optical interleaver capable of separating closely spaced optical channels with minimal cross-talk.